The present invention relates to a vehicular hydraulic brake system with slip control, wherein a booster piston which is preferably pressurizable by means of an auxiliary pressure is slidably guided in a housing, wherein a second piston is adapted to be coupled with the piston in the direction of actuation and pressurizable in opposition to the direction of actuation, wherein the two piston parts form within the housing a chamber of varying volume associated with at least one wheel brake, and wherein the annular surface of the housing which is pressurizable in opposition to the direction of actuation is connected to its pedal-near annular surface via a throttling channel and the pedal-near effective surface is smaller than the pedal-remote effective surface.
In a brake system according to the prior art a booster piston which is preferably pressurizable by means of an auxiliary pressure is displaced in the direction of actuation when the braking pedal is depressed, the working chamber confined inside the housing by means of the piston parts thereby being reduced while the pressure is being increased. The pressure built up in the working chamber thereby propagates to at least one wheel brake monitored by slip control electronics.
When critical slip values occur at the vehicle wheel assigned to the wheel brake, a switch valve which is located in a hydraulic connection between a hydro-pneumatic pressure accumulator and a annular housing chamber confined by the pedal-remote annular surface of the positioning sleeve is switched over. Upon actuation of the switch valve, the pressure of the pressure accumulator is transmitted to a change-over valve which first separates the housing chamber confined by the larger effective surface of the housing from the unpressurized supply reservoir and then connects it directly to the pressure accumulator. A pressurization of this larger volume annular housing chamber results in a displacement of the positioning sleeve in opposition to the direction of actuation, the braking pedal being displaced, if necessary, into the brake release direction.
On pressurizing the housing chamber connectable to the pressure accumulator, a housing annular chamber which is confined by a smaller effective surface of the housing is also pressurized, the pressure built up in the respective housing annular chamber being higher, due to the differing volumes, and being able to gradually adapt to that in the larger volume annular housing chamber connectable to the pressure accumulator via a throttling channel. As a result, the reset movement of the positioning sleeve is delayed, so that an abrupt reset action will be prevented.
In the brake system described above, a stroke limitation is required during the control action in order to ensure a minimum volume in the working chamber of the brake pressure generator, since a failure of the pressure accumulator or components of the slip control electronics, even during the control operation, cannot be excluded. Even in an incident such as this, occuring during a control operation, it must be ensured that the working chamber is pressurizable such that the prescribed minimum deceleration of the vehicle is achieved, and that solely by the force acting upon the brake pedal. Practice has shown that it is sufficient to limit the actuating travel of the booster piston to approximately half of its total stroke. Consequently, there is no need to reset the booster piston during the control operation when it has effected less than half of the maximum stroke.
It is, therefore, an object of the present invention to improve a brake system initially referred to, with the aid of constructively simple means, such that a reset action of the braking pedal and/or the booster piston will only set in when the minimum reserve stroke of the brake force generator is exceeded during the control operation.